I doubt noobs would know how to turn it off, that was my point ... Making noobs do something assumes they know how to do it.
If someone doesn't know how to toggle auto-cast, they need a tutorial, not a feature to help them. If they're that lost, then the issue of micromanagement and ease thereof is rather moot.
Ironically, when I was a newbie and first encountering this autocast behavior, I actually was caught offguard when my mothership automatically casted malice as soon as I added to it, depriving me of the ability to use shield regeneration at that critical moment. This isn't necessarily a feature beneficial to beginners, either. Just because you're new doesn't mean you won't want to manually control your units.
Therefore, I suggest making it so the skilled players are required to do something.
That wouldn't be a problem if there weren't consequences for the players who don't want auto-cast. There are no consequences for someone who does want autocast and has to toggle it to get it; it's one click. The way this is set up right now, it's taking away strategic choice from the player.